Shaftless propulsion systems are an alternative to the traditional shafted propeller-and-seal systems used by marine vehicles, such as undersea vehicles. Disadvantages of traditional shafted propeller-and-seal systems include mechanical vibrations and noise that propagate down the shaft and become radiated noise. The shaft itself adds considerable weight to the marine vehicle, and leaks may develop due to shaft seal wear. Shafted systems also are labor and cost intensive to both install and maintain.
A shaftless propulsion system (SPS) typically uses an electric motor integrated within the hull of the marine vehicle. One such shaftless propulsion system is described in U.S. Pat. No. 5,078,628 to Chester A. Garis, Jr. (hereinafter referred to as the '628 patent). As shown in FIGS. 1 and 3 of the '628 patent, the shaftless propulsion system is installed within the body 10 of the marine vehicle. As shown in FIG. 2 of the '628 patent, the shaftless propulsion system includes a rotor 30 and one or more stators 26, 28. In the system of the '628 patent, the two stator disks 26, 28 are secured to the body (hull) 10 of the marine vehicle and, in essence, become part of the body 10. The stators 26 and 28 and rotor 30 are disk shaped, without a drive shaft or other mechanism occupying their center. A blade hub 24 with blades 22 is attached to the rotor 30. The propeller blades 22 extend beyond the circumference of the vehicle housing. The system also includes a shroud 14 with rib supports 18 and openings 20 to accommodate rotating blades 22.
In the system of the '628 patent, the rotor 30 and the stators 26, 28 are located outside of the vessel body 10 and are cooled by the surrounding water. They operate in water and at submergent water pressure, thus requiring no complex seals between the motor and the body 10 of the vessel. The rotor 30 is positioned between the two stators 26, 28 and is accompanied by a journal bearing 48 that is positioned between the rotor 30 and the body 10. As shown in FIG. 3 of the '628 patent, the shaftless propulsion system is installed between two sections 38, 40 of the pressure hull which, after installation, are bolted and secured together. In particular, the rotor 30 is journal mounted on a central housing 44, and the rear body section 40 of the vessel can be detachably connected to the front section 38 of the vessel by a bolting flange 46 extending from the central housing 44. The rear section 40 can be connected to the flange 46 by bolts 48.
In addition to the positioning of the stators and rotors, the '628 patent discloses a cooling system that also is installed within the hull of the marine vehicle. The journal bearing 48 is water cooled and lubricated. Cooling fluid such as filtered seawater is pumped with an onboard pump through a piping system that is sea-connected. The cooling fluid is pushed out through the main fluid conduits 50 and the secondary fluid conduits 52 which lead to the journal bearing 48 and thrust bearing assemblies 54.
U.S. Pat. No. 5,509,830 to Chester A. Garis, Jr. (hereinafter referred to as the '830 patent) describes an improved cooling and lubricating system for a marine propulsor such as the propulsor described in the '628 patent. The blades of the propulsor are used to bring in cooling water, thereby dispensing with the need for a separate pump and piping system. Also, U.S. Pat. No. 5,286,116 to Chester A. Garis, Jr. (hereinafter referred to as the '116 patent) describes an improved motor and bearing assembly for use in an axial gap electric motor of a marine propulsor such as the propulsor described in the '628 patent. The bearing assembly of the '116 patent is configured to permit the stator-to-rotor gap and the bearing clearance to be adjusted after final assembly.
One of the most significant disadvantages of the shaftless propulsion system taught by the '628, '830, and '116 patents is that the stators are located within the body of the undersea marine vehicle and are attached to the hull. The problem with the system of the '628 patent stems from the fact that undersea marine vehicles compress with increasing depth during a dive. When the undersea marine vehicle dives, the hydrostatic pressure outside the hull exceeds the atmospheric pressure inside the hull. This pressure difference compresses not only the hull, but also the rotor bearing attached to the hull and the stators attached to the hull. Typical depths can compress the hull by several inches in diameter, and the compression increases with increasing depth. Although the diameter of the stators and rotor bearings of the shaftless propulsion system are reduced due to compression during a dive, the diameter of the rotor remains unchanged because the rotor is not attached to the hull and is completely immersed in the seawater surrounding the marine vehicle. As a result, the gap between the rotor and the rotor bearing increases as the depth of the marine vehicle increases. At or near the surface of the ocean, the gap between the rotor and the rotor bearing is small. At increased depth, the hull (and rotor bearing) radius decreases due to hydrostatic pressure, enlarging the gap between the rotor and the rotor bearing. The enlarged gap leads to several adverse consequences.
When the gap between the rotor and the rotor bearing increases, the rotor is able move so that it is no longer concentric with the rotor bearing and the stators. This non-concentric movement can cause several problems. For example, the windings of the stator are no longer parallel with the rotor windings, which causes reduced efficiency of the motor and reduced output of the propulsor. In addition, the surface area of the rotor bearing that is in contact with the rotor is reduced, which leads to increased bearing wear. Also, the play from the enlargement of the gap between the rotor and the rotor bearing may cause the rotor to have impact collisions with the rotor bearing when the marine vehicle changes direction. The impact collisions between the rotor and rotor bearing cause further wear of the bearing surface and also act as a source of radiated noise. Furthermore, if the space between the outer circumferential edge of the rotor blades and the shroud is not sufficient, the non-concentric movement due to an enlarged gap between the rotor and the rotor bearing may lead to collisions between the rotor blades and the shroud. Such collisions could damage the rotor blades and the shroud, and also would act as a source of radiated noise.
In summary, the shaftless propulsion system taught by the '628 patent could be rendered noisy, damaged and/or non-functional due to the dive-induced non-concentric movement of the rotor that is caused by the enlarged gap between the rotor and the rotor bearing experienced during the dive.
Another disadvantage of the shaftless propulsion system as taught by the '628, '830, and '116 patents lies with the maintenance of the system and the repair/replacement of worn or damaged parts. In order to repair or replace the shaftless propulsion system of the '628 patent, the marine vehicle must be placed in drydock. For example, the marine vehicle may need to be disassembled and/or the hull cut open in order to access the stators and rotor bearing. Placing a marine vehicle in drydock is expensive and time consuming. With respect to the problem of adjusting the rotor bearing without drydocking the vessel, the solution offered by the '116 patent is not adequate. The '116 patent attempts to address the goal of not taking the undersea vehicle apart to adjust the bearing by providing a bearing in which the gap is adjustable by manually adjusting nuts and screws, where the screws penetrate through the pressure hull. Such penetrations through the pressure hull are disadvantageous for obvious reasons.